Morning everyone

I hope this is the correct forum for this question.

I have a customer that has an old SBS 2003 server and is using exchange for their emails.  They have a single customer that can't send them any emails.

I have looked into the issue, and turned on all the logging I can think of both in exchange and the spam software we use (Exchange Toolbox from Jam Software) and as far as I can tell the emails just don't get to the SBS server.

Over the last couple of years we haven't had any issues with other customers (I’m basing that on that fact that we haven’t had any reports from users that they haven't received an email they were expecting) and I’ve sent emails from multiple email accounts from multiple providers without issues. The company that is having trouble sending to my customer are getting NDR reports but not being an expert in email I’m unsure which server is bouncing the mail. 

I’m pretty sure it’s not the SBS 2003 machine though. Example of an NDR (emails and servers changed to protect the innocent):

This message was created automatically by mail delivery software.   A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:   User@SBSServer     No such user   ------ This is a copy of the message, including all the headers. ------   Return-path: <user@sendingcompany.co.nz> Received: from ???.???.???.???.adsl.xtra.co.nz ([???.???.???.???] helo=SERVER.SendingCompany.local)                 by worm.hosts.net.nz with esmtpsa authed as sendingcompany.co.nz  (TLS1.0:RSA_AES_128_CBC_SHA1:128)                 (Exim 4.80)                 (envelope-from < user@sendingcompany.co.nz >)                 id 1VjaUG-00058N-9G                 for User@SBSServer; Fri, 22 Nov 2013 09:04:28 +1300 Received: from SERVER.SendingCompany.local ([????::????:????:???:????]) by  SERVER.SendingCompany.local ([????::????:????:???:????]) with mapi id 14.01.0438.000;  Fri, 22 Nov 2013 09:04:23 +1300

While it seems to me that the issue is at the senders end, perhaps an Incorrect DNS setup or reading the wrong MX record or something like that, I don’t want to go back with the standard “no issue at our end problem must be at your end” response. And of course if the issue is at my end I’d like to get it fixed.

So finally onto my question J  is there a test I can do to investigate this further or steps I can take to narrow it down so I can tell the sender what the issue is? Or correct it if it’s at my end?

Some additional info: There are three domains on this SBS server and the customer reports that they can’t send to any of them.  The three domains have different MX records but obviously end up pointing to the same IP address. Also the customer used to be with Telecom for their emails (a couple of years ago) and I have checked with telecom that they haven’t got any settings still configured for the domain in question. Thanks for any help you can give.

Craig